Reach is the total number of people who have taken action — viewed or clicked — on the content in your feed.
What does it tell you?
Subscribers is a measure of how many people are subscribed to your feed. At any given time, you can expect that a certain percentage of this subscriber base is actively engaging with your content and this “Reach” measurement provides this additional insight.
Additionally, there may be people viewing your content beyond your known subscriber base. For example, they may view your content on a feed search engine or news filter site.
Reach aggregates both of these groups, providing an accurate and useful measurement of your true audience.
How is it reported within FeedBurner?
On the Dashboard and Subscribers pages, it looks like this: